Internationally, the usage of so-called biometric characteristics is postulated increasingly, for example in the US-visit and European programs.
Am aim of these programs is to increase the security, for example for airline passengers, and to recognize terrorists as early as possible.
For the acquisition of the biometric data, characteristics of the face, of the iris and of the finger are used preferably. Because of the high recognition rate and the invariability of the characteristics during the process of aging and because of the distinguishability even for uniovular twins, the characteristics of the finger are especially suited.
Especially for surveillance of persons at a border crossing, the biometric characteristics of a person demanding admittance have to be matched with existing, very large databases which can contain many millions of entries. In doing so, it has to be ensured that the biometric data being deposited in a fraud resistant way on a passport is in accordance with the entering person. As these characteristics have to be identified in a so-called 1:N search out of millions of comparison informations, in particular of criminal search databases, in case of finger characteristics a recognition problem of the following type arises:
The smaller the scanned surface of the finger which is to be identified the more inaccurate the results of the 1:N search are, because the number of characteristics, so-called minutiae, are not sufficient for a definite identification. Studies have shown that recognition methods using finger sensors which give only a simple impression of the finger reach a recognition rate of 80% to 85% only, because of the not entirely acquired finger surface. Such methods which are called flat finger impressions are predominantly used in civil areas. For protection against crime, a recognition rate that low is insufficient.
Due to this, unrolled fingerprints have always been used in official criminology, where the person to be registered has to dye the fingers first and to roll them onto paper to be able to reproduce the entire surface of the finger from nail to nail.
Newer methods are known, too, where so-called live-scanners are utilized which replace the intricate dyeing, wherein the finger is rolled across a glass plate and the contrast due to a disabled total reflection arising thereof is used for optically imaging by means of a camera. With this method, it is possible to significantly increase the recognition rate of the 1:N identification in large databases to more than 98%.
With this method it is disadvantageous that the person to be registered has to roll the finger on an underlay. Normally, this results in problems with unexperienced users, wherein the fingerprints become smeared. Additionally, because of the different application pressure the fingers can deform differently or yield bad contrasts. Furthermore, it has been observed that, besides of a lack of contrast, in particular in case of dry skin, grease marks are left which can also be recognized in a subsequent identification. To avoid this, the registration underlays consisting of glass have to be cleaned after each use as a general rule. Because a supervisor is required for an optimal accuracy, these methods cannot be used reasonably at automated control sites, for example so-called kiosks in the aforementioned entry programs.
Systems are known which enable to image a finger touchlessly.
For example, it is known from EP 1 073 988 B1 and WO 99/56267 respectively to image a finger touchlessly, wherein the formation of the contrast is based upon exploiting features of polarized light. Thereby, the insufficient light efficiency turns out to be disadvantageous, because the polarization filter prevents an effective usage of the photons and therefore the efficiency factor is low. Furthermore, distortions caused by the imaging process are not compensated and result in the recognition of wrong minutiae very often, which is caused by shadowing to the dermal ridges in the utilized optical path of illumination.